In recent years, for example, portable electronic appliances such as a digital video camera are increasing rapidly, and a great importance is placed on the performance of a secondary battery mounted on these electronic appliances. As one of these secondary batteries, there is a so-called lithium ion type battery.
In addition, in such portable electronic appliances described above using a secondary battery as a power source, many of them have the function of indicating the remaining amount of the battery mounted thereon. Particularly, in a lithium ion secondary battery, since it has properties that the voltage of the battery cell drops gently and linearly except right after the start of discharge and right before the end of discharge, the remaining amount of the battery can be relatively, accurately predicted and indicated.
However, although the secondary battery including the lithium ion type has the capacity that is decided for every battery cell, it has characteristics that the capacity changes depending on the temperature for use. For example, when the battery is used at low temperature, the internal impedance of the battery cell becomes high. Thus, the voltage drop becomes large when it is intended to carry current at the same current value, and the capacity of the battery cell is reduced. In addition, also in the case in which the number of times of charges and discharges is increased because of continuous use, it has characteristics that the capacity is reduced. This is because the repetition of charge and discharge causes the battery cell to be deteriorated, and the usable capacity is reduced.
Then, in a secondary battery before, such functions are provided that the temperature of the battery cell and the number of times of charge and discharge are detected, and the remaining amount of the battery is corrected in accordance with the detected values. For example, there is a secondary battery in which the correction value of the remaining amount corresponding to the detected value of the temperature of the battery cell is set to each of the numbers of times of charge and discharge, and the computation error of the remaining amount of the battery is reduced (for example, see JP-A-2005-147815 (paragraph numbers [0017] to [0026], and FIG. 1)). In addition, in the secondary battery before, it is general that the number of times of charges is detected when the voltage of the battery cell rises and drops around a predetermined threshold.
Moreover, today, in order to more accurately predict the capacity of a battery, such battery packs are commercially available that detection circuits for the voltage and current of the battery cell and microcontrollers for various correction processes are accommodated in the same package as a battery cell. Such battery packs have functions that communicate with a device to be a discharge load and output various detected values thereinside to the device, in which the device having received these detected values computes the remaining amount of the battery as a usable time and indicates it. For example, inside the battery pack, the summed value of current in the battery cell and the decay rate of the capacity based on the temperature and on the number of times of charges and discharges and others described above are detected, in which these values are received on the device side and the remaining amount of the battery can foe computed accurately.